The invention concerns a low pressure fuel system with fuel preheating for an air-compressing, injection internal combustion engine.
Because, at very low temperatures, diesel fuels precipitate paraffin particles which rapidly block the fine filter and adversely affect the driving operation, measuring as provided in a known manner for heating the fuel in fuel systems. Thus, for example, in a known design (ATZ 10 Automobiltechnische Zeitschrift 85 (1983) 11, p. 671), a switch-over valve controlled as a function of fuel temperature is located in a supply main leading to the feed pump downstream of a suction filter designed as a prefilter. This switch-over valve makes it possible to divert the fuel induced by the feed pump through a heating-water/fuel heat exchanger at low temperatures but removes the diversion to the heat exchanger at higher fuel temperatures so that the fuel can flow directly to the feed pump without diversion. Operational faults can scarcely be avoided, however, because the prefilter tends to become blocked at low temperatures because of paraffin precipitations in the fuel.
A remedy is provided by the arrangement in accordance with German Patent Specification 3,538,360, in which the switch-over valve and the prefilter, which form one structural unit, are arranged in such a way that, because of the special flow path, the prefilter also falls within the compass of the heated fuel.
Both the arrangements previously mentioned, have a common disadvantage. Immediately after starting of the internal combustion engine, there is still not a sufficient quantity of cooling water heat available for heating the diverted fuel. The danger of blockage cannot therefore be excluded.
The use of electrical preheating, previously known from German Offenlegungsschrift 3,034,730, is also a disadvantage to the extent that excessive demands are made of the battery capacity of the vehicle.
Finally, a solution is proposed in the as yet German unpublished patent application P 36 31 579.6-13 in which, by arranging an overpressure valve in a bypass around the fine filter, the fuel first flows into the filter casing of the prefilter before the time when the opening pressure is reached because of the accumulation of paraffin and the fuel then passes into the injection pump via the bypass.
This solution, however, does not make it possible to avoid impurities arriving unhindered in the injection pump or even in the internal combustion engine during the period when the bypass is open.
The object of the invention is to avoid the disadvantages described by a simple measure.
This object is achieved by providing shut-off valve in the return main of a high pressure injection pump and controlling the shut-off valve by a thermostatic element in the supply main to the injection pump.
The effect that only the quantity of fuel required for combustion in the internal combustion engine is passed through the filter is achieved by shutting off the fuel return when the temperature is low.
The smaller quantity of fuel, corresponding to the instantaneous fuel consumption, is heated more rapidly. Operational interruptions due to blocking of the filter are avoided. The filtering capability is ensured.
An advantageous measure for the invention results by arranging a temperature sensing element, for example a thermostat, which triggers the shut-off valve as a function of temperature, in the supply main.
The insertion in the supply main of a temperature responsive expansion material operating element, which controls the shut-off valve steplessly, should be considered as a preferred measure of the invention.
A useful extension of the invention is obtained by providing an overflow valve in the return main downstream of the steplessly operating shut-off valve. Because of the special position of this overflow valve, a pressure balance is achieved on the fuel supply side and fuel return side of the shut-off valve so that load is removed from the expansion material operating element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.